1. Field of the Invention
This invention relates to a thermal process for recovering oil from a subterranean, viscous oil-containing formation. More particularly, this invention relates to a thermal method of recovering oil from a viscous oil-containing formation, especially a highly viscous tar sand deposit, employing a selective injection system for injecting a thermal fluid into the bottom portion of the formation and a sequence of manipulative steps with steam and hot water to obtain maximum heat utilization and oil recovery from a spaced-apart production well completed in the upper portion of the formation.
2. Background of the Invention
Increasing worldwide demand for petroleum products, combined with continuously increasing prices for petroleum and products recovered therefrom, has prompted a renewed interest in the sources of hydrocarbons which are less accessible than crude oil of the Middle East and other countries. One of the largest deposits of such sources of hydrocarbons comprises tar sands and oil shale deposits found in Alberta, Canada, and in the Midwest and western states of the United States. While the estimated deposits of hydrocarbons contained in tar sands are enormous (e.g., the estimated total of the deposits in Alberta, Canada is 250 billion barrels of synthetic crude equivalent), only a small proportion of such deposits can be recovered by currently available mining technologies (e.g., by strip mining). For example, in 1974, it was estimated that not more than about 10% of the then estimated 250 billion barrels of synthetic crude equivalent of deposits in Alberta, Canada was recoverable by the then available mining technologies. (See Synthetic Fuels, March 1974, pages 3-1 through 3-14). The remaining about 90% of the deposits must be recovered by various in-situ techniques such as electrical resistance heating, steam injection and in-situ forward and reverse combustion.
Of the aforementioned in-situ recovery methods, steam flooding has been a widely-applied method for heavy oil recovery. Problems arise, however, when one attempts to apply the process to heavy oil reservoirs with very low transmissibility such as tar sand deposits. In such cases, because of the unfavorable mobility ratio, steam channelling and gravity override often result in early steam breakthrough and leave a large portion of the reservoir unswept. The key to a successful steam flooding lies in striking a good balance between the rate of displacement and the rate of heat transfer which lowers the oil viscosity to a more favorable mobility ratio.
Copending application filed July 20, 1982, Ser. No. 400,178, by Shu et al discloses a thermal method for the recovery of oil from a subterranean, viscous oil-containing formation, steam in an amount ranging from 0.3 to 0.5 pore volume and an injection rate within the range of 4.0 to 7.0 bbl/ac.-ft. is injected into the formation via an injection well completed in the lower 50% or less of the formation and fluids including oil are recovered via a spaced-apart production well completed in the upper 50% or less of the formation. The injection well is then shut-in for a variable time and thereafter a predetermined amount of hot water or low quality steam is injected into the formation via the injection well in an amount ranging from 0.3 to 0.10 pore volume and at an injection rate of 1 to 2.0 bbl/day/ac.-ft. The method is applied to viscous oil-containing formation in which either naturally occurring or induced communication exists between the injection well and the production well in the bottom zone of the formation. The injection well and production well are spaced apart 400 to 750 feet.
Copending application filed Nov. 12, 1981, Ser. No. 320,236, by Shu et al discloses a thermal method for the recovery of oil from a subterranean, viscous oil-containing formation, wherein a predetermined amount of steam in an amount not greater than 1.0 pore volume is injected into the formation via an injection well and oil is produced from the formation via a production well. The injection well is then shut-in for a variable time to allow the injected steam to dissipate its heat throughout the formation and reduce oil viscosity while continuing production of oil. A predetermined amount of hot water or low quality steam in an amount not greater than 1.0 pore volume is injected into the formation with continued production but avoiding steam breakthrough. Thereafter, production is continued until there is an unfavorable amount of water or steam in the fluids recovered.
Applicant's copending application filed concurrently herewith, Ser. No. 447,596 relates to an improved thermal system for effectively recovering oil from subterranean formations such as tar sand deposits utilizing a deviated injection well extending into the lower portion of the formation and a production well completed in the upper portion of the formation combined with manipulative steam flooding.
Applicant's copending application filed concurrently herewith, Ser. No. 447,731 relates to a method for recovery of oil from a viscous oil-containing formation not greater than 2,500 feet in depth employing a horizontal fracture formed in the lower portion of the formation through the injection well, a spaced-apart production well completed in the upper portion thereof, and manipulative steam flooding.
Accordingly, this invention provides an improved thermal system for effectively recovering oil from subterranean formations such as tar sand deposits utilizing a selective injection well and production well completion combined with manipulative steam flooding.